Method and apparatus for controlling the composition of the combustible mixture of an engine

ABSTRACT

An internal combustion engine is provided with an ion current sensor in the exhaust conduit. The sensor may be simply a spark plug across which is applied the vehicle battery voltage. The operation of the engine at various values of mixture composition yields characteristic curves for the ion current and for the fluctuations in the ion current as a function of the air number. These characteristic data are used to provide set point values against which the prevailing ion current is compared. If a set point value is exceeded, the fuel-air mixture is adjusted accordingly by an integral controller.

BACKGROUND OF THE INVENTION

The invention relates to a method and an apparatus for determining thecomposition of the mixture fed to an internal combustion engine bysensing the magnitude of operational engine parameters.

Several methods are known for regulating the composition of a fuel-airmixture in an internal combustion engine so as to obtain exhaust gasesas free from toxic components as possible. For example, the engine maybe regulated to operate with a stoichiometric mixture having an airnumber λ=1.

For this purpose, operational parameters of the engine are sensed, forexample the exhaust gas composition, with the aid of an oxygen sensorwhich measures the presence of free oxygen in the exhaust gas of theengine. Such known oxygen sensors have the advantage of generating aclear and regular control signal during the transition from ahyper-stoichiometric to a hypo-stoichiometric mixture and vice versawhen the air number λ traverses the value 1.0 but they have thedisadvantage of being fairly expensive since they have a platinumcontaining surface material and their life time is relatively shortbecause of thermal and mechanical loads.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore a principal object of the invention to provide a methodusing a transducer which senses an operational engine parameter which isa measure of the exhaust gas composition of the engine. The transducermust deliver a clearly defined and steady control signal and be ofrelatively simple construction as well as being capable of withstandingthe relatively rough conditions in an internal combustion engine.

It is a further object of the invention to provide an apparatus forapplying this method to the control of the exhaust gas composition of anengine. The method and apparatus are intended to maintain a flawlessoperation of the engine, to obtain exhaust gases as free from toxiccomponents as possible while at the same time avoiding theabove-mentioned disadvantages.

These and other objects are attained according to the invention byproviding a per se known ion current detector located downstream of theexhaust valves in the exhaust gas system of the engine which measuresthe magnitude of post combustive reactions in the gas leaving thecombustion chambers of the engine. An electronic regulator or processorthen uses the information in the ion current from the ion current sensorto provide appropriate control signals for changing the composition ofthe fuel-air mixture admitted to the engine.

In an application of the above method, the ion current is a controlledvariable which is compared with a command variable within a controllerand, depending on the deviation from the command value or nominal value,the controller sets a final control element which changes thecomposition of the operational mixture and/or of the exhaust gas of theengine. The apparatus for attaining the desired objects and forperforming the method of the invention provides at least one ion currentdetector in the exhaust gas of the engine and a comparison circuitfollowed by an integral controller which influences a final controlelement which changes the composition of the fuel-air mixture and/or theexhaust gases.

By using the method and apparatus of the invention, the fuel-air mixturefed to the engine may be altered with respect to its components, i.e.,fuel, oxidizer, especially air, and possibly recycled exhaust gas, inthe desired proportions so as to obtain the most desirable exhaust gascharacteristics. The exhaust gas composition may also be changed inindependent manner by changing the ignition timing.

The invention will be better understood as well as further objects andadvantages thereof become more apparent from the ensuing detaileddescription of an exemplary embodiment of the invention and severalembodiments of processing circuitry for the ion current sensor signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing the integrated ion current I at the outletof the cylinder as a function of the air number during one operationalcycle of an engine piston;

FIG. 2 illustrates the disposition of an ion current sensor in theexhaust system;

FIG. 3 is a schematic diagram of a first exemplary embodiment of aprocessor circuit;

FIG. 4 is a schematic diagram of a second exemplary embodiment of aprocessor circuit having a relatively low sensor supply voltage;

FIG. 5 is a schematic diagram of a third exemplary embodiment of aprocessor circuit with a high sensor supply voltage; and

FIG. 6 is a schematic diagram of a fourth exemplary embodiment of aprocessor circuit which uses the changes of the integrated ion currentsin several sequential cycles in the cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Introduction

The various steps of the method and the elements of the apparatus forcarrying out this method and for maintaining optimum operation of theengine will now be described. It is known that, when the operationalmixture of an engine is hypo-stoichiometric, corresponding to airnumbers λ<1, not all the oxygen is actually used in the cylinder of theengine and a small percentage of free oxygen is still present in theexhaust gases expelled from the cylinders. It is further known that whenthe mixture supplied to an externally ignited engine is increasinglyleaned out, the combustion process is displaced to a greater extent intothe domain of the expansion stroke of the piston until, finally, theretakes place a well-defined post combustion process within the exhaustsystem of the engine. This fact may be ascribed to the increasingportion of uncombusted oxygen in the exhaust gas when the mixture isexcessively leaned out. The magnitude of this post combustive reactionmay be detected by means of an ion current sensor, located as shown inFIG. 2 downstream of the exhaust valves within the exhaust system of theengine.

B. Preferred Embodiments

Turning now to FIG. 2, there is shown in simplified form a portion of acylinder 2 of an internal combustion engine. Moving within the cylinder2, in the customary reciprocating manner, is a piston 3. During itsfirst downward stroke, the piston aspirates the fuel-air mixture throughan induction tube 4 and an open inlet valve 5 into the combustionchamber 6. In known manner, an exhaust gas conduit 7 leads from thecombustion chamber to an exhaust system, not further shown. An outletvalve 8 controls the communication between the combustion chamber 6 andthe exhaust conduit 7 in known manner. As a major feature of theinvention, there is disposed within the exhaust line 7 and downstream ofthe exhaust valve 8 an ion current sensor 9. In the present example,this sensor is a commercial spark plug with an insulated electrode 11cooperating with a ground electrode 10 or even with the grounded wallsof the exhaust manifold 7.

If a potential is applied across the electrodes 10 and 11, an ioncurrent will flow as soon as an ionization of the gases between theelectrodes occurs due to a post-combustion process. A relatively lowvoltage suffices to obtain an ion current, for example the normalbattery voltage of the vehicle in which the engine is operating. Thenormal high voltage insulation exhibited by commercial spark plugs wouldnot be necessary in principle; a simple insulated electrode mountedwithin the exhaust line 7 would be sufficient. In particular, a groundelectrode such as illustrated in FIG. 2 is not necessary because theentire wall of the exhaust system serves as a ground electrode. If theion-receiving surface of the electrode is enlarged and/or the potentialis increased, the ion current increases for a constant degree ofionization of the surrounding gas.

FIG. 1 is a diagram illustrating the behavior of several components ofthe exhaust gas as a function of the air number λ. In particular, thediagram illustrates the magnitude of uncombusted hydrocarbons (HC), andresidual oxygen (O₂); the hydrocarbon curve exhibits a minimum in theregin of air numbers around λ>1.0 and this minimum indicates the regionof the minimum fuel consumption of an engine. The curve labeled I is theion current flowing through an ion sensor of the type described above.This curve I has a rapid rise in he domain of μ>1 and continues with asmaller slope and finally experiences another steep rise in the vicinityof the minimun of the curve HC. The first portion of this curve isdetermined by the post reaction of the oxygen which begins to rise inthis region. The latter part of the reaction is determined by the rapidincrease of the HC curve. In this region, the exhaust gas containssufficient combustible components which then react with the ample amountof oxygen present. Both of the steep increases of the ion current curvemay be used, according to the invention, for controlling the fuel-airmixture admitted to an internal combustion engine. For this purpose,optimum operational points on the curves are chosen lying in a richregion, i.e., when the engine is to deliver high torque, or in a leanregion for a more economical operation of the engine.

The illustrated curve I is the result of integrated ion currents duringone operational cycle of the engine. It is necessary to integrate thecurrent over one cycle because the maximum of the post-combustionprocess occurs at very different times and, during a single cycle, veryprominent changes in the reaction and hence of the ion currents takeplace. The curve S also shown in the diagram of FIG. 1 illustrates thefluctuations of sequential integrated ion currents per cycle. This curvealso exhibits a well-defined increase in the region of the HC minimum atair number λ>1, i.e., at the lean running limit of the internalcombustion engine. This increase in the curve S may also be used forcontrolling the fuel-air mixture as may the relative fluctuations of theintegrated ion currents ΔI/|I|, i.e., the fluctuations with respect tothe instantaneous value of the measured ion current.

The first increase of the characteristic curve I in the full-load domainof the engine, i.e., when λ<1 and corresponding to a rich mixture,becomes more pronounced the closer the ion current sensor is physicallylocated to the exhaust orifice of the exhaust gases from the combustionchamber 6 near the valve 8. This fact may also be caused by theturbulent admixture of the exhaust gases directly behind the outletvalve which favors the homogenization of uncombusted fuel and oxygen.The outlet valve itself acts as a flame supporter. The foregoing remarksillustrate that an ion current sensor may be used to definecharacteristic operating points of the engine both in the lean operatingregion as well as in the full-load domain if the ion currents aresupplied to a controller which adjusts the mixture accordingly. It hasbeen found that when the exhaust gas recycle rate is increased and ifthe ignition timing α_(z) is changed in the direction of the top deadcenter, there occurs a well-defined increase of the ion current and ofits fluctuations in a manner similar to that which occurs when the airnumber is changed to extremely lean mixtures. This effect is also due todisplaced combustion events. Thus, the same control variables may beused for regulating the exhaust gas recycle rate and the ignition timingα_(z). The circuitry to be described below for changing and adjustingthe fuel-air mixture may also be used for adjusting these parameters.The ion currents may also be used for monitoring the engine operation,for measuring the air number as well as for test stands if suitableelectronic controllers and processors are employed.

Instead of using the values illustrated by the curves I and S in FIG. 1,which are values integrated over an operational cycle of the engine, itis also possible to use the maximum value of each of these parametersfor use as a control variable.

FIG. 3 is a schematic diagram of an electronic processor circuit forreceiving the currents from the ion current sensor and using them tocontrol the fuel-air mixture of an engine. In this first exemplaryembodiment, each cylinder outlet has associated with it a sensor withelectrodes 11, 11', 11", which are joined together. Opposite theseelectrodes are the ground electrodes 10, 10', 10" which could also beparts of the exhaust gas tubing. A well-insulated and shielded line 14leads from the electrodes of the sensors to a comparator circuit 15. Thecomparator circuit 15 includes a grounded capacitor 16 whose other sideis connected to the line 14, a connection line 17 leading to a controlcircuit 19 and the inverting input of a comparator 20.

The capacitor may be periodically charged over the connection line 17.For this purpose, the connection line 17 includes a diode 21 and alimiting resistor 22. The other input of the comparator is connected toa reference value indicator 24 which delivers to that input of thecomparator a constant or variable voltage acting as a command value. Thecommand value may be changed in dependence on other operationalparameters, for example the rpm and the throttle valve position of thecarburetor.

The circuit 19 which determines the degree of charging of the capacitorincludes a pulse-shaping circuit 26 which receives rpm-dependent pulsesfrom an ignition element of the engine, for example the distributor 27.The rpm-dependent pulses are converted into rectangular pulses atrpm-dependent frequency. These pulses are fed to a first monostableflip-flop 28 one of the outputs of which is connected to the input of asecond monostable multivibrator 29 and the complementary output of theflip-flop 28 which signals the quiescent state of the flip-flop 28 isconnected through a line 30 with a bistable flip-flop 32. The output ofthe flip-flop 29 related to its flipped-over state is coupled to aconnection line 17.

The output of the comparator 20 is connected to the set input of thebistable flip-flop 32. The circuit 32 includes in known manner two NORgates 33 and 34, the input of the gate 34 beinng connected to the line30 and the output being connected through a diode 35 and a resistor 36whith the inverting input of an operational amplifier 37. The output ofthe operational amplifier 37 is coupled back to its inverting input by acapacitor 38 which thereby imparts to the amplifier an integralbehavior. The diode 35 is connected so as to permit positive current toflow to the operational amplifier. A diode 39 is connected in theopposite direction in series with a resistor 40 between the invertinginput of the operational amplifier 37 and the line 30 via a wire 42.

The other input of the operational amplifier 37 is connected to avoltage divider consisting of series resistors 44 and 45 to whichbattery potential is applied. The output of the integral controller 41formed by the operational amplifier 37 and the integrating capacitor 38is coupled, for example, with a control element 46 belonging to themetering mechanism of the fuel mixture of the engine. A settingmechanism of this type influences the metering of fuel in known mannerby means of a carburetor or by fuel injection nozzles or it controls theamount of bypass air added to a fuel-air mixture. In the same manner,the setting element 46 may affect the quantity of recycled gases for thepurpose of exhaust gas detoxication or it may adjust an element of thetiming mechanism. The system operates in the following manner. The pulsetrain of rpm-dependent frequency produced in the pulse former 26produces output pulses of the same frequency from the first monostableflip-flop 28 having a well-defined pulse width whereas the output of thesecond monostable flip-flop 29 produces pulses which are displaced inphase with respect to those from the first flip-flop 28 by one pulsewidth. These pulses, which alternate in amplitude between zero and thebattery potential and which are of defined width, serve to charge thecapacitor 16 periodically through the diode 21. In the time between thepulses, the capacitor may discharge in accordance with the magnitude ofthe prevailing ion current through electrodes 11, 11', 11'". The diode21 prevents a discharge through the line 17 while a discharge throughthe input of the comparator 20 is prevented by the high input impedance.When the potential across the capacitor drops below a predeterminedthreshold which is defined by the command value provided by the commandvalue generator 24, the comparator 20 switches over and produces asignal corresponding to a logical 1 at the set input of the bistableflip-flop 32, i.e., at the NOR gate 33. The complementary output of thefirst flip-flop 28 then transmits through the line 30 pulses changingfrom logical 1 to logical 0 to the reset input of the bistable flip-flop32, i.e., at the NOR gate 34. During the extent of these pulses but onlywhen the set input has a logical 1, may the flip-flop be switched backso that, during the time that the comparator 20 produces a logical 1,the output of the NOR gate 34 again exhibits a logical 1. However, the1-signal at the output of the comparator 20 returns to 0 at the instantwhen the capacitor 16 has been fully charged through the connection line17.

Inasmuch as the non-inverting input of the operational amplifier 37 liesat some voltage in between the full battery voltage and 0, i.e., betweenthe logical 1 and 0, no return current flows through the return line aslong as the line 30 experiences the logical 1-signal from thecomplementary output of the first flip-flop 28. However, once the line30 experiences the logical 0-signal, a current does flow there whosemagnitude is defined by the value of the resistor 40. These conditionsprevail in each case for the duration of the flip-over process of thefirst monostable flip-flop 28. If during that time and because of amissing 1-signal from the comparator 20, the output of the bistableflip-flop 32 has the value 0, then a current flows out of theoperational amplifier 37 through the return line 42 to the line 30. Ifon the other hand the bistable flip-flop 32 exhibits a logical 1, thecurrent flowing through the resistor 36 is determined by the value ofthat resistor 36 and that current later branches off in one partialcurrent flowing through the return line 42 and another partial currentflowing into the operational amplifier 37. For this purpose, theresistor 36 has a smaller value than the resistor 40. The switchingcircuit just described provides that the integral controller 41integrates in one or the other directions dictated by the signal fromthe comparator 20. The output voltage at the operational amplifier 37 islinear because of the feedback through the integrating capacitor 38 fromthe output of the operational amplifier 37 back to its inverting input.The slope in a particular integrator will be determined by the magnitudeof the corresponding resistors 40 and 36. The unsymmetric disposition ofthe integral controller 41 produces average operational points whichcause a shift to larger or smaller values of λ than would correspond tothe particular value of λ during the ion current increase.

In other words, the above-described installation uses the ion currentcorresponding to a particular operational state of the engine andtransforms it into a corresponding voltage at a capacitor whosemagnitude is between two well-defined potentials and whose remainingvoltage is then compared in a comparator 20. The capacitor also servesat the same time as an interference suppressor against voltages whichare produced for example in the high-impedance, shielded line 14 throughinduction. Any voltage peaks occurring there would otherwise cause arapid switching of the comparator and would prevent a precise controlprocess. The processor circuit illustrated in FIG. 4 is substantiallysimilar to that of FIG. 3. Except for the comparator 15', the system ofFIG. 4 is substantially identical to that in FIG. 3 so that these partsare to be taken from the previous description. In contrast to theexemplary embodiment of FIG. 3, the present embodiment dispenses with acomparator 20 which must have a very high input impedance so as not tofalsify the relatively weak ion currents occurring when battery voltageis used as the driving potential for the ion currents. In its place,there is provided a transistor 47 and a continuously settable voltagedivider 48 within the connection line 17. The tap of the voltage divider48 is connected to the base of the transistor 47 and may be adjusted bythe command value generator 24. Thus, the command value generator mayadjust the voltage applied to the base of the transistor in dependenceof any other parameter. The emitter of the transistor 47 is suppliedwith battery voltage through a connection to the line 17 between thecontrol circuit 19 and the voltage divider 48. A collector resistor 49is connected between the collector of the transistor 47 and ground. Thesignal is taken off between the collector and the collector resistor andis fed to the set input of the bistable flip-flop 32, i.e., to the inputof the NOR gate 33.

In this exemplary embodiment, which is suitable for a PNP transistor, itis the recharging current of the transistor that is used as a measure ofthe ion current which had passed through the sensors instead, aspreviously, the voltage occurring during the discharging process.Depending on the degree of discharge of the capacitor in the timeelapsing between the recharging pulses provided by the control circuit19, a charging current of varying magnitude flows through the voltagedivider during the charging process. The voltage drop changesaccordingly, and so does the voltage taken off at the tap of the voltagedivider 48. When the switching voltage of the transistor is exceeded,the transistor switches on and the collector exhibits battery voltagewhich is then fed to the bistable flip-flop 32 as a logical 1. Incontrast to the exemplary embodiment of FIG. 3, the first and secondmonostable flip-flops 28 and 29 are controlled in synchronism by thepulse shaper 26 so that there is no phase shift in the pulses theyproduce. However, as in the exemplary embodiment of FIG. 3, the durationof the switch, i.e., the pulse width of the second flip-flop 29 isshorter than that of the first. Furthermore, as before, the bistableflip-flop 32 switches over only when the complementary output of thefirst flip-flop has a 0-signal and when the collector of transistor 47has a logical 1.

This exemplary embodiment brings the advantage that a transistor may beused instead of a high impedance comparator. Furthermore, the chargingcurrent is substantially easier to define than the capacitor voltage.

A variant of the exemplary embodiment of FIG. 4 is illustrated in FIG.5. This example is substantially similar to the above-described examplesbut includes additional circuitry for supplying a higher than batteryvoltage to the capacitor 16 and the electrodes 11, 11', 11". For thispurpose, the control circuit 19" is modified so as to place between thecomplementary output of the flip-flop 29 and the line 17 a circuitincluding a transistor 51 whose base is connected to the complementaryoutput of the monostable flip-flop 29 via a voltage divider consistingof two resistors 52 and 53. The collector of the transistor 51 isconnected to the positive battery voltage and its emitter is grounded.The collector resistor is a coil 55 of high inductivity and theconnecting line 17 is connected between the junction of the coil and thecollector of the transistor 51 leading to the capacitor 16. A Zenerdiode 56 is connected in parallel with the transistor 51 between theline 17 and ground preventing current flow to ground.

The circuit just described operates substantially in the same manner asthat of FIG. 4. Thus, the monostable flip-flops 28 and 29 aresimultaneously flipped over by the pulse from the pulse shaping stageand thus both generate a pulse of well-defined width at rpm-dependentfrequency. The time constant of the second flip-flop 29 is also shorterthan that of the first flip-flop 28. As long as the complementary outputof the second flip-flop 29 carries a voltage, the transistor 51 conductsand a constant current flows through the coil 55. The connection line 17is at 0 potential thus the transistor 47 remains blocked. When thesecond flip-flop 29 switches over, the base of the transistor 51 goes to0 and blocks this transistor. This interruption produces an inverseinductive potential which is limited in magnitude by the Zener diode 56.During this time, the connecting line 17 carries a high positive voltagewhich causes the capacitor 16 to be recharged. At the same time, asalready described with respect to the previous example, the transistor47 is switched on. Thus, the input of the NOR gate 33 in the bistableflip-flop 32 receives a voltage serving as a logical 1 whose magnitudeis defined by the voltage divider connected between the ground and thecollector of the transistor 47 and consisting of the resistors 58 and59. This signal causes the bistable flip-flop 32 to be swtiched over ifa pulse is present at the line 30.

The just described exemplary embodiment brings the advantage that asubstantially greater measuring potential may be used which results in ahigher sensitivity of the whole system. Furthermore, the use of atransistor 47 in place of a comparator makes this installationeconomically more favorable and less subject to interference.

The exemplary embodiment illustrated in FIG. 6 is a variant of thecircuit of FIG. 3. The output stage is built in the same way as that ofthe example of FIG. 3 so that any information concerning the comparator20, the bistable flip-flop 32 and the integral controller 41 may be hadfrom the description of FIG. 3. Identical elements carry the samereference numerals. In deviation from the exemplary embodiment of FIG.3, the present variant has a switch 61 in the connecting line 62 whichconnects the line 17 with a comparator 20. Following the switch 61 is acapacitor 63 which is connected through a resistor 64 with the invertinginput of an operational amplifier 65. The output of that amplifier inturn is connected to an adapting resistor 66 at the inverting input of asecond operational amplifier 67. Finally, a line 68 leads from theoutput of the first operational amplifier 65 via a diode 69 to theoutput of the second operational amplifier 67. A further diode 70 isinserted in parallel to the diode 69 between the output of the secondoperational amplifier 67 and the junction with the line 68. Behind thejunction with the line 68, the output of the second operationalamplifier 67 is connected to the inverting input of the comparator 20.The inverting input is connected to the positive potential via aresistor 71. The non-inverting input of the comparator 20 is connectedwith a command value generator 24, just as in the example of FIG. 3, bymeans of which the threshold of the comparator can be adjusted. Thediodes 69 and 70 are both connected to prevent positive current flow tothe comparator 20.

In this example, the switch 61 is a field effect transistor which isperiodically closed by the control circuit 19''' in advance, by onephase, of the charging process of the capacitor 16.

The control circuit 19''' includes three sequential monostableflip-flops, the first of which is supplied with rectangular pulses ofrpm-dependent frequency by the pulse shaper 26. The output of the firstflip-flop 28 is connected to the input of a second flip-flop 73 whoseoutput, in turn, is connected to a third monostable flip-flop 74. Thecomplementary output of the first flip-flop 28 branches off to a clockline 30 leading to the bistable flip-flop 32. The output of the secondmonostable flip-flop 73 controls the switch 61 through a line 75 and theconnecting line 17 branches off from the output of the third monostableflip-flop to the capacitor 16 and includes the diode 21 and the resistor22, as was the case previously.

The just described circuit operates as follows. The control circuit19''' produces at its output pulses of defined width and rpm-dependentfrequency and these pulses are phase shifted by one pulse width. Thismeans that the capacitor 16 can be charged through the connecting line17 only if the switch 61 has previously been opened. During this openingtime, the capacitor 16 is connected in parallel to the capacitor 63which is grounded through the first operational amplifier 65 and itsnon-inverting input. Depending on the magnitude of the differentialcharge, an equalizing current flows between the two capacitors in thedirection of the prevailing potential difference. Accordingly, theoutput of the first operational amplifier 65 also carries a voltagewhich, depending on the direction of the equalizing current, is carriedeither through the line 68 and the diode 69 or through the secondoperational amplifier 67 which inverts it and through the diode 70 tothe inverting input of the comparator 20. Depending on whether thisvoltage pulse exceeds the set command value at the comparator, thiscomparator delivers a 1 or a 0 to the bistable flip-flop 32 whichprocesses it in the same manner as previously described, for examplewith respect to FIG. 5. When the switch 61 is closed, the phase-shiftedpulse from the third monostable flip-flop recharges the capacitorwhereas the previous value is stored in the capacitor 63. Depending onthe degree of ionization of the exhaust gases, a smaller or largerdischarge current flows through the ion current sensors during the timeuntil the switch 61 is opened. When the switch 61 is opened, thepotenial on the capacitor 16 may be higher, equal to or smaller thanthat on the capacitor 63. The difference of this voltage between theprevious and the subsequent cycle of the engine is expressed in theamount and direction of the equalizing current. Actually, use is madeonly of the magnitude of the equalizing current and, when apredetermined magnitude is exceeded, the integral controller alters theaction of the fuel metering system in the direction of a rich mixture.It is this circuit according to FIG. 6 which processes the data in curveS in the diagram of FIG. 1 for regulating the fuel mixture fed to theengine. It will be seen that very lean operational states of the engineare thus subject to control. In comparison to the regulation of the ioncurrent, the present system provides the advantage that the fluctuationsbegin to increase in the neighborhood of the air number λ=1. This systemcan also be used very advantageously for controlling the exhaust gasrecycle rate and the ignition timing angle. Other possibilities forcontrol are given, as in previously described examples, by changing thecommand value generator in dependence on engine variables such as rpm,induction tube pressure or throttles valve angle, as well as coolingwater temperature.

The above-described method and apparatus thus provides manifoldpossibilities for controlling the operation of an engine and fordiagnosing the composition of the exhaust gases in such an engine. Inits operation, the apparatus uses a cheap sensor not subject tointerference and a relatively simple control system.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other embodiments and variantsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed is:
 1. A method for controlling the fuel-air mixture ofan internal combustion engine, said engine including combustionchambers, inlet conduits leading to said combustion chambers and exhaustconduits leading from said combustion chambers to the atmosphere,comprising the steps ofplacing an ion-current sensor in the exhaustconduit for measuring an ion-current; generating a nominal value of theion-current; comparing the magnitude of the ion-current with the nominalvalue and generating a signal representing the difference between themeasured value of the ion-current and said nominal value; andperiodically integrating a signal derived from the signal representingthe difference between the measured value of the ion-current and saidnominal value, especially during one working cycle of a piston of theengine, and altering the fuel-air mixture of the engine in dependence onthe result of the integration.
 2. A method as defined in claim 1,wherein said ion-current sensor is placed in the immediate vicinity ofthe exhaust valves of the engine.
 3. A method as defined by claim 1,wherein the result of integration is used to change the ignition timingof the engine.
 4. A method as defined by claim 1, wherein the generatednominal value is changed in dependence on other operational parametersof the engine.
 5. A method as defined by claim 1, wherein the step ofcomparing uses the increasing ion-currents resulting from theintegration in the region of the air number λ≦1 as a controlledvariable.
 6. A method as defined by claim 5, wherein the controlvariable is the increase of the integrated ion-currents at air numbersλ≦1 and λ>1 and wherein the transition between these two operationaldomains is effected by a pre-control of the air number.
 7. A method asdefined by claim 1, including the further step of comparing the resultsof said integration with one another.
 8. A method as defined by claim 1,wherein the step of comparing includes comparing the fluctuations ofsequential values of said integration with respect to the instantaneousvalue I/|I|.
 9. An apparatus for controlling the fuel-air mixture of aninternal combustion engine, said engine including at least onecombustion chamber, inlet conduit means leading to said combustionchamber, exhaust conduit means leading from said at least one combustionchamber to the atmosphere, and fuel mixture preparation means, saidapparatus comprising:at least one ion-current sensor, disposed withinsaid exhaust conduit means for measuring the amount of ionized exhaustgas and for generating a sensor signal indicative thereof; means forgenerating a nominal value; a comparator connected to said at least oneion-current sensor and to said means for generating a nominal value forcomparing said sensor signal with the nominal value for generating asignal representing the difference between the measured value of theion-current and said nominal value; an integral controller, commanded bysaid comparator, to periodically integrate a signal derived from thesignal representing the difference between the measured value of theion-current and said nominal value, especially during one working cycleof a piston of the engine, and a final control element, controlled bysaid integral controller, for adjusting said fuel-mixture preparationmeans as a function of the results of integration.
 10. An apparatus asdefined by claim 9, wherein said integral controller includes anoperational amplifier having an integrating capacitor connected betweenits output and its inverting input and wherein there is disposed behindthe integral controller a bistable flip-flop whose reset input isconnected with a control circuit by means of which the bistablemultivibrator is periodically called upon to process the output signalof the comparator.
 11. An apparatus as defined by claim 10, wherein theinverting input of said operational amplifier is connected through afirst resistor and a diode to the control circuit and via a secondresistor and an oppositely connected diode with the output of saidbistable multivibrator, the second resistor being of a smaller valuethan said first resistor.
 12. An apparatus as defined by claim 9,wherein said comparator uses the ion-current as control variable bytesting the remanent voltage or the charging current of a periodicallychargeable second capacitor.
 13. An apparatus as defined by claim 12,wherein said comparator includes said second capacitor one of theelectrodes of which is connected to ground whereas the other electrodeis connected to at least one of said ion-current sensors the otherelectrode also being connected with a threshold switch as well as withsaid control circuit via a line containing a diode; whereby saidcapacitor may be charged by pulses of preferably rpm-dependentfrequency.
 14. An apparatus as defined by claim 13, wherein theoperating voltage of the ion-current sensor is the battery voltage of amotor vehicle.
 15. An apparatus as defined by claim 13, wherein saidthreshold switch includes a comparator element connected with a nominalvalue generator.
 16. An apparatus as defined by claim 13, wherein saidthreshold switch includes a transistor the collector emitter path ofwhich is connected in parallel with said second capacitor and the baseof which is connected to an adjustable voltage divider for receiving thecharging current through said second capacitor and acting as thethreshold generator, the collector resistor of said transistor beingconnected with said integral controller.
 17. An apparatus as defined byclaim 13, wherein said threshold switch includes a comparator elementconnected with a threshold indicator and wherein the connecting linebetween said second capacitor and said comparator element includes aswitch which is activated by said control circuit by a pulse which isadvanced by one phase with respect to the charging pulse for said secondcapacitor and wherein the circuit further includes a differentiator anda rectifier subsequent to said switch.
 18. An apparatus as defined byclaim 17, wherein said differentiator includes an operational amplifierthe inverting input of which is connected with said switch via aresistor and a third capacitor.
 19. An apparatus as defined by claim 17,wherein said rectifier includes a further operational amplifier theoutput of which is connected to a diode and a further parallel diodeconnected between the inverting input and the output of said furtheroperational amplifier.
 20. An apparatus as defined by claim 13, whereinsaid threshold switch further includes a comparator element connectedwith a nominal value generator and wherein said control circuit includesa pulse shaping circuit connected with the ignition system of theengine, said pulse shaping circuit controlling a first monostableflip-flop behind which is connected a phase shifting second monostableflip-flop the output of which is connected to said second capacitor andwherein the complementary output of said first monostable flip-flop isconnected to the reset input of said bistable flip-flop.
 21. Anapparatus as defined by claim 13, wherein said threshold switch furtherincludes a comparator element connected with a nominal value generatorand wherein said control circuit includes a pulse shaping circuitcoupled to the ignition system of the engine, said pulse shaping circuitcontrolling a first and a second monostable multivibrator, wherein theoutput of said second monostable multivibrator is connected to saidsecond capacitor and wherein the complementary output of said firstmonostable multivibrator is connected with the reset input of saidbistable flip-flop.
 22. An apparatus as defined by claim 21, wherein thecomplementary output of the quiescent position of said second monostablemultivibrator is connected via a voltage divider with the base of atransistor having an inductive coil as collector impedance, thecollector of which is connected via a Zener diode to ground and is alsoconnected to said comparator.
 23. An apparatus as defined by claim 13,wherein said threshold switch includes a comparator element and anominal value sensor and wherein the connection between said secondcapacitor and said comparator element includes a switch which isactuated by said control circuit and where there is connected beyondsaid switch a differentiator and a rectifier, and wherein said controlcircuit includes a pulse shaping circuit connected to the ignitionsystem of the engine, said pulse shaping circuit actuating a firstmonostable multivibrator and, in sequence, a second monostablemultivibrator for the generation of mutually phase shifted pulses and,behind that, a third monostable multivibrator, the complementary outputof said first multivibrator being connected to the reset input of saidbistable multivibrator and the output of said second monostablemultivibrator being connected to said switch, while the output of saidthird monostable multivibrator is connected to said second capacitor.24. An apparatus as defined by claim 9, wherein said ion-current sensoris a spark plug.